Integrated circuits are typically fabricated using photolithographic processes that employ a photo mask and an associated exposure tool to transfer a circuit image or other such pattern onto a substrate, such as a silicon wafer coated with photoresist. The completed integrated circuit includes many such patterned layers, and the quality of the resulting integrated circuit is highly dependent upon precise alignment of the layers, one to another. Thus, it is very important to ensure that the alignment processes are well characterized, capable, and in control.
The degree to which one layer is properly registered to another layer is typically measured by inspecting the spatial relationship between two features, commonly called registration marks, one of which resides on a first of the two layers, and a second of which resides on a second of the two layers. Typically, the two marks are considered to be a single registration mark when combined. However, for the sake of readily referring to the separate parts of the registration mark, they are designated as separate marks herein.
The linear distances between one or more given elements of the two features are measured and compared to an ideal. The ideal is typically measured by how well one mark is centered in regard to the other mark, such as is depicted in FIGS. 1A-1D. The greater the discrepancy between the measured linear distances and the ideal, the poorer the alignment between the two layers. Information relating to the measured accuracy or inaccuracy of the alignment marks is used to make corrections to the fabrication process to improve the overlay accuracy between the two layers.
Alignment inaccuracies can arise from various sources during the alignment process. These sources include imperfections in the exposure tool, distortion of the alignment marks during subsequent processing of the substrate, and interactions between the exposure tool and distorted marks. Similar inaccuracies can arise during the measurement process. For example, measurement inaccuracies can result from imperfections in the measurement tool, distortion of the registration marks, and interactions between the measurement tool and the registration marks.
Such measurements of the alignment marks and of the registration marks of different layers are typically made at two different times during a photolithographic fabrication cycle. First, an alignment measurement is typically made by the exposure tool which exposes the images onto the substrates, as it aligns a succeeding mask layer to a preceding mask layer. In making such an alignment, the exposure tool adjusts the registration of the image to the substrate, until its sensors indicate that the registration, or in other words the alignment, of the image to the substrate is perfect, or at least within a predetermined maximal error, at which point the exposure tool exposes the image onto the substrate.
The second measurement is typically conducted after a few intervening process steps have been performed, such as developing and hard baking the photoresist on the substrate. At this point, a measurement tool is typically employed to measure the offsets between sets of registration marks, being either the same marks used by the exposure tool to determine alignment, or other features on the patterned surface of the substrate. The measurement tool also reports a value, which may or may not be equal to the ideal offset, and which further may or may not be equal to the offset reported by the exposure tool.
Thus, each of the two different instruments used to measure the offsets between the two layers makes the implicit assumption that it has provided an absolutely correct set of results. However, the two sets of results may not agree, thus raising the understanding that one of the sets of results, and possibly both sets of results, are inaccurate. The question then exists, which data set is used to determine whether the alignment and exposure has been properly made, and how should the data be used, if at all, to adjust the exposure tool for future exposures. In other words, which instrument is to be believed, and what is to be done with the data.
The invention advantageously achieves improved correction of alignment inaccuracies by taking into account inaccuracies associated with both the exposure tool and the measurement tool.